Add OBJDIR reference to arduino-mk-vars.md

Signed-off-by: tinyladi <ladislas@weareleka.com>

HISTORY.md

@@ -10,6 +10,7 @@ I tried to give credit whenever possible. If I have missed anyone, kindly add it
 - Fix: Add "gpio" to the list of isp that don't have a port. (Issue #165, #166) (https://github.com/sej7278)
 - Fix: Add "-D__PROG_TYPES_COMPAT__" to the avr-g++ compiler flags to match IDE. (https://github.com/sej7278)
 - New: Create `Makefile-example-mk`, a *real life* `Makefile` example, to be used as a reference. (https://github.com/tinyladi)
+- Tweak: Add `OBJDIR` to `arduino-mk-vars.md` (https://github.com/tinyladi)
 
 ### 1.3.1 (2014-02-04)
 - Fix: BUNDLED_AVR_TOOLS_DIR is now set properly when using only arduino-core and not the whole arduino package. (https://github.com/sej7278)

arduino-mk-vars.md

@@ -129,6 +129,24 @@ Defaults to 'libraries' directory within user's sketchbook.
 
 Optional.
 
+### OBJDIR
+
+**Description:**
+
+Directory where binaries and compiled files are put.
+
+Defaults to `build-$(BOARD_TAG)` in your `Makefile` directory.
+
+**Example:**
+
+```Makefile
+OBJDIR = /path/to/my/project-directory/bin
+```
+
+**Status:** *Optional*
+
+----
+
 ### ALTERNATE_CORE
 Non-standard core for Arduino-unsupported chips like the ATtiny.